Any device that requires polarized light uses one or more polarizers. Polarizers have many industrial applications. For example, polarizers may be utilized in electro-optical modulators and laser subsystems. In essence, a polarizer eliminates an undesirable light component of a first polarization, and allows a desirable light component of a second polarization to pass through.
Of particular interest is the use of polarizers as in-line modules in optical fibers. Previously known in-line polarizers typically comprise an assembly with a first lens following a first optical fiber for collimating the light emerging from the fiber. The collimated light then passes though a polarizer plate and is then focused by a second lens into a second optical fiber. The main disadvantage of this type of polarizer is that it is relatively expensive and difficult to construct. Furthermore, the lens-based polarizer interrupts the optical fiber leading to optical loss and undesirable reflection. Finally, the lens-based polarizer introduces a device into the fiber that is much larger than the fiber, thereby causing potential space and size issues.
One attempt to solve the above problems was the development of another in-line fiber polarizer that was constructed by wrapping the optical fiber in several loops around a circular member before allowing the fiber to continue on its way. This arrangement eliminated some of the drawbacks of the previously known lens-based polarizer—for example, this was a true in-fiber device that did not interrupt the fiber with a much larger device. However, the coil-based polarizer suffered from another significant drawback—the coil element around which the fiber needed to be wrapped was typically many centimeters in diameter. Thus, while not as unwieldy as a lens-based polarizer, the coil-based polarizer was still very bulky and difficult or impossible to use in many applications.
A novel in-fiber polarizer, that advantageously solved all of the problems of the prior art polarizers was disclosed in a commonly assigned U.S. Pat. No. 6,721,469, issued on Apr. 13, 2004, and entitled “Chiral In-Fiber Adjustable Polarizer Apparatus and Method” (hereinafter the “Adjustable Polarizer patent”), which is hereby incorporated by reference in its entirety. That novel adjustable polarizer worked with circularly polarized light and utilized a fiber component that functioned as a quarter-wave plate to convert circular polarization into linear polarization over a relatively narrow frequency band. The fact that polarization conversion only happens across a narrow frequency band, is one of the chief limitations and drawbacks of quarter-wave plates and quarter-wave plate-type devices. In addition, since most practical applications utilize linearly polarized light (for example transmitted through standard polarization-maintaining fibers), the polarizer disclosed in the Adjustable Polarizer patent required conversion of incoming light into circularly polarized light prior to entering the polarizer.
In addition, previously known in-line polarizers must be spliced in-line with one or two optical fibers when in use resulting in various fiber coupling related issues such as insertion loss, diameter/material mismatch, etc. In addition, most previously known in-line polarizers have to be fabricated from special preforms. Finally, in-line polarizers with narrow diameter regions are typically limited in maximum power of the signal being transmitted therethrough as a result of a “leading effect”—primarily, the Stimulated Brillouin Scattering (“SBS”) effect. The SBS effect is highly undesirable in typical narrow diameter polarizers, because it results in a backward propagating wave of the same linear polarization as the incident (i.e., “fundamental”) wave, creating undersirable interference and limiting maximum incident signal power.
It would thus be desirable to provide an in-line polarizer that does not interrupt an optical fiber with a larger structure and that is capable of operating with an unpolarized light input. It would further be desirable to provide an in-line polarizer that does not require a preform and that can be easily fabricated to desired specifications and characteristics, by modifying a region of a particular optical fiber to enable the desired polarizer properties therein. It would also be desirable to provide an in-line polarizer that substantially minimizes or eliminates the SBS effect.